Systems comprising an electronic sensor unit for determining a relative position with the aid of encoder elements of an encoder are found in many fields. Such systems serve for example for determining a relative position between two members, of which generally one member is fixedly positioned and the other is movable relative thereto at least along one axis. Such a relative position may be e.g. the position of a machine component on a linear axis or rotation axis. Such systems are accordingly found in apparatuses such as coordinate measuring machines (CMM), geodetic apparatuses, robot arms, articulated arms, motors or hydraulic actuators. In this case, the sensor unit is fitted to one member, and the encoder is fitted to the other member. For determining a relative position, a sensor of the sensor unit detects at least one encoder element of the encoder. In order to be able to cover the entire possible range of relative movement, either the encoder extends over this range of movement, e.g. in the form of an elongated material measure as a fixedly positioned member, and the sensor unit is movable and designed for detecting a narrowly delimited region of the encoder. Alternatively, the sensor unit is fixedly positioned and extends over the region to be detected and the movable encoder, e.g. a magnetic target, has a narrowly delimited expansion. As a further alternative, the sensor unit is fixedly positioned and designed for detecting a narrowly delimited region and an elongated encoder moves relative to the sensor unit.
In some systems of this type, the sensor unit used has at least two individual sensors. In this case, the sensors are spaced apart from one another in a defined manner by means of being fixed to a substrate, e.g. that of a printed circuit board, wherein the value of the sensor distance is stored in a control and evaluation unit of the system. The sensors are configured such that they can each detect at least one encoder element. EP 1195880 A1 discloses such a system comprising an encoder and a sensor unit, which uses two sensors of a sensor unit that are spaced apart from one another in order to determine error values for the encoder elements of a movable encoder with the aid of the known sensor distance.
Environmental influences and/or aging phenomena can cause a change in the distance between the sensors in the case of sensor units of this type. One such environmental influence may be the action of heat, for example, which leads to an expansion of the substrate, the extent of which expansion is dependent on the coefficient of thermal expansion (for short: CTE) of the substrate material. By way of example, if a printed circuit board serves as a substrate, heat develops on account of the electric currents flowing there when the sensor unit is used, as a result of which the printed circuit board expands. In the case of conventional printed circuit boards comprising FR-4 laminate, the coefficient of thermal expansion is approximately 15·10−6K−1, such that a sensor distance of e.g. 20 mm changes by 3 μm in the case of a temperature change of 10K. Highly precise measured value determinations can be made more difficult or impossible by such changes in the sensor distance.
DE 3818044 A1 discloses a measuring device for incremental position determination between a sensor unit and an encoder. The encoder is designed as an elongated material measure with precision graduation. The sensor unit has a substrate, to which at least two photoelectric line sensors are fitted, wherein the sensors are spaced apart in the direction of the extent of the material measure. The material measure and the substrate consist of the same materials or at least of materials having the same coefficient of thermal expansion, wherein the materials are preferably transparent glass ceramics or quartzes having a negligible coefficient of thermal expansion. The material measure has a position code having alternating incremental and absolute sections, designed as sequences of light-transmissive and light-opaque increments. The distance between the sensors is adapted to the distance between the absolute sections in such a way that at least one absolute code section is detectable by at least one of the sensors in each positioning of sensor unit with respect to material measure. The position is determined from the detected increments. What is disadvantageous about the measuring device disclosed is that for highly precise measurements both sensor unit and encoder have to be embodied highly precisely and sensor distance and section distance between the absolute code sections have to be accurately coordinated with one another. What is disadvantageous, moreover, is that all code elements detected by all sensors have to be used for ascertaining a relative position. The questions concerning what absolute value is required for the coefficient of thermal expansion of the substrate material in order that it can be regarded as negligible, and how the sensors should be fixed to the substrate in order to determine the sensor distance, are neither addressed nor disclosed in DE 3818044 A1.